releasetools: Remove the support for generating verify package.

We used to have "--gen_verify" that would generate a verify package for
non-A/B devices. Since a) we don't have active users; b) it works with
non-A/B only; c) we have better alternatives (e.g. using fastboot to
compute the partition checksum), this CL removes the support.

Test: `m dist`
Change-Id: Ib4a2c8c2a0394b54e66c106089aa52a0b900f034